The present invention is related to rotation sensing apparatus. More particularly, the invention is directed toward a high accuracy, dual-track target wheel rotational sensor.
High accuracy rotational sensors are known which utilize dual-track target wheels and a dual-element sensor. Each section of such a target wheel is axially adjacent the other along the rotational axis thereof, and each section has disposed radially adjacent thereto one of the two elements. Such target wheel arrangements are particularly advantageous when employed, for example, as part of a rotating member such as an internal combustion engine crankshaft for determining angular position information therefor. A known target wheel for such a sensor arrangement includes substantially complementary, or mirror image, geometries. That is to say, a tooth location on one section is adjacent a slot on the other section. Output signals from the two elements are complementary and provide for high accuracy tooth edge detection with relatively simple differential signal processing. An example of such a sensor apparatus may be found in U.S. patent application Ser. No. 08/262,097, filed Jun. 20, 1994, assigned to the assignee of the present invention.
Referring to FIGS. 1A and 1B, related rotational sensor arrangements are shown being viewed from a substantially tangential vantage point with respect to a rotating member. In each Figure, a dual-element magnetoresitive sensor is generally labeled 11 and comprises individual magnetoresistive elements MR1 and MR2, and a bias magnet 13. Although not separately illustrated, a ferromagnetic shim may be located between the bias magnet 13 and the individual MR elements. The MR sensors are generally balanced having positively correlated, magnitudinal equivalence such that equivalent flux density through each individual element produces a substantially equivalent MR response. Such dual-element MR sensors are generally well known, an exemplary one of such sensors being found in U.S. Pat. No. 4,926,122 also assigned to the assignee of the present invention. Also in each Figure, a rotational axis is labeled (Ar) and corresponds to an axis of rotation of a member (not illustrated) such as an engine crankshaft. Coupled to the rotating member is a target wheel 15 forming a pair of immediately adjacent tracks, one track on each respective side of plane `P` which is orthogonal to the axis of rotation and which hence appears as a line in the Figure separating left and right portions of the receptive views. Each target wheel 15 is characterized by teeth on each section generally axially adjacent to discontinuities on the other section save perhaps for small, axially-adjacent, discontinuous sections at the angular interfaces between teeth on opposite axial sides of the target wheel. Examples of such target wheels are described in the aforementioned co-pending U.S. patent application Ser. No. 08/262,097.
In each FIG. 1A and 1B, the target wheels 15 are illustrated in a rotational position (solid line) whereat a tooth and a discontinuity toward the left and the fight of the plane P, respectively, are axially aligned beneath the respective MR sensor. Alternatively, another rotational position is exemplified by the broken line whereat a tooth and a discontinuity toward the right and the left of the plane P, respectively, are axially aligned beneath the MR sensor. As the target wheel rotates, these two positions essentially alternate thus producing substantially angle-coincident inverse resistive changes to the individual MR elements which are sensed and processed to produce an angle indicative signal. Generally, this is accomplished with differential signal processing or comparator means, one such exemplary sensing and processing means being disclosed in the aforementioned co-pending U.S. patent application Ser. No. 08/262,097.
In all applications, a certain degree of axial movement of the rotating member relative to the sensor will occur. Accommodation of such axial movement is required in order to minimize the angular inaccuracy in minor cases and to ensure operativeness of the sensor in extreme case. Such accommodation has been accomplished through widening each track of the target wheel and/or widening the axial spacing between the two sensing elements to ensure acceptable levels of performance. Of course, space and placement limitations may greatly restrict the degree to which such accommodations may be implemented.
The two views of related sensors in FIGS. 1A and 1B generally graphically describe conventional management of axial play affecting the MR sensor outputs. FIGS. 2A thru 2C, illustrating individual MR element outputs and angular information derived therefrom, will also be referred to herein in exposition of the management of axial play illustrated in and described with respect to FIG. 1A and 1B. In each FIG. 1A and 1B, it is understood that the respective MR sensor is mounted net to a major body; in the case of the exemplary application of crankshaft sensing, the engine block provides the preferred mounting provision. Therefore, any axial movement of the rotating member, the engine crankshaft in the example at hand, is therefore with respect to the engine block and MR sensor. The preferred sensor/target wheel alignment is one wherein the centerline 16 located mid-way between individual MR elements MR1 and MR2 corresponds to the interface or plane P between the two tracks of the target wheel. Such relative positioning preferably corresponds to the position of the crankshaft at the center of its axial tolerance. In other words, assuming a total axial tolerance for movement of .DELTA., alignment of the crankshaft as described would allow maximum movement to either side of the sensor centerline 16 of substantially .DELTA./2. In each FIG. 1A and 1B, an mount of axial displacement of .delta. is illustrated wherein .delta..ltoreq..DELTA.2.
Desired sensor performance is accomplished in either of the arrangements of FIGS. 1A and 1B when the MR sensor elements are symmetrically aligned over the target wheel such as previously described. Sensor response with such ideal alignment is represented in FIG. 2A wherein the respective MR elements produce outputs having inverse symmetry with the respective outputs intersecting at the desired rotation angle .theta.net.
Generally, the width of the target wheel is application specific and established with such considerations as mechanical strength and available location and space on the crankshaft. For axial tolerances that are relatively small with respect to an available width of the target wheel as illustrated in FIG. 1A, and further assuming that in that Figure .delta.=.DELTA./2 such that the axial displacement is illustrated at a maximum or worst case, the axial spacing between MR1 and MR2 ensures that for all axial displacements the MR elements remain over respective tracks. However, even with such provision, MR outputs would vary from ideal substantially as shown in FIG. 2B. An examination of the outputs exposes a shift in the level of the output of MR2 when located over a respective discontinuity due to ferrite effects from the adjacent track moth. A resultant shift in the intersection of the respective outputs occurs at rotation angle .theta..delta.. Therefore, the sensor angular error is substantially .+-.(.theta.net-.theta..delta.).
For axial tolerances that are relatively large with respect to an available width of the target wheel as illustrated in FIG. 1B, and continuing with the assumption that .delta.=.DELTA./2 such that the axial displacement is illustrated at a maximum or worst case, the axial spacing between MR1 and MR2 is relatively wide to provide adequate peak to peak output and a maximum axial displacement operating band. Closer placement would result in increased adjacent track ferrite effects and less tolerance to smaller axial displacements. However, even with such provision, substantial angular error would result due to shifting of the outputs intersection and potentially loss of intersection where, as illustrated, the axial deviation is so great as to place one MR element (MR2) closer to a tooth of the adjacent track than the other MR element (MR1) is to that same tooth. In this case, MR outputs would vary from ideal substantially as shown in FIG. 2C and result in total loss of an angular position signal since such is, as described, a result of differential processing of the MR outputs.